Method of conditioning aluminum material for spot welding



Patented Dec. 19, 1944 METHOD OF CONDITIONING ALUMINUM MATERIAL FOR SPOT WELDING Don E. Stevens, South San Francisco, Calif., as-

signor to Hammond Aircraft Company, South San Francisco, Califo No Drawing.

Application September 6,1943, Serial No. 501,431

Claims. (c1. 134-28) This invention relates to a method of conditioning aluminum sheets for spot welding, and has for its main object a more effective Way of removing aluminum oxide from the surfaces of aluminum sheets to prepare them for spot welding than heretofore, and which method is relatively quick, simple and economical.

Heretofore, tests have been established for spot welds of aluminum sheets. In formulating the tests and requirements, it has been usual to clean the surfaces of the plates with steel wool. A series of spot welds are then made and are tested for strength and to be satisfactory, the welds cannot vary more than a certain predetermined percentage relative to a. pre-established mean. The use of steel wool for cleaning all plates would be excellent, if practical, but it is obvious that such method of cleaning is prohibitive under modern conditions from the stsgldpoint of labor and materials. Where steel wool is used in orderto set up a mean for strength of weld, the average variation from the means is about 3 /z%, which is well within the acceptable-range. Whil the range varies with requirements, a reasonable one permitsa maximum variation.

In the aircraft industry, where spot welding of lit aluminum sheets preparatory for spot welding comprises first immersing the sheets in a bath of about 1 /2% to a 2/z% solution of hydrofluoric acid (HF) for a period of about 20 to about 30 seconds, preferably the former. This appears to break the aluminum oxide coating on the sheets,

although the complete removal of the oxide and any other impurities is the next step. The time here involved is relatively critical, so as to prevent deposits of fluorides on the sheets in too great an amount.

After the sheets are removed from the hydrofluoric acid solution, they are then immersed in a bath of about a 4% to 6% solution of hydrochloric acid .(HCl) for a period of from about 60 seconds to about 120 seconds, after which they may be washed free of the acidand are ready for welding.

As a single standard for solutions and times that may be considered preferable for most conditions, the initial bathmay be substantially a. 2 solution of hydrofluoric acid, and the time the sheets are subjected thereto may be substantially 20 aluminum sheets is most prevalent, various acid I solutions have been tried as baths for the aluminum sheets to clean them for welding. A hydro-. fluoric acid solution alone has been used for some time, but it has not been considered successful,

since the spot welds are inconsistent and the' burning of spots is excessive. Deviations from the established mean strength are not uncommon. At the present time, apart from this invention, the use of a phosphoric acid solution with a wetting agent of butyl cellosolve is considered as being about the best, but here again, deviations of as high as 20% relative to the established mean are found to occur, and the-time of immersion of the sheets in the bath is from eight to twenty minutes.

seconds. The second bath may be-substantially a 6% solution of hydrochloric acid and the time the sheets are subjected thereto may be substantially 90 seconds. The hydrochloric acid solution very effectively removes I the hydrofluoric acid solution and the solubles therein, and also the By using the method'herein described, the average deviation from the established mean,.

where aluminum sheets have been immersed in succession in the two solutions, is about 4%.,

' Practically all welds after the treatment of the sheets by this invention are well within the limits' of rigid standards, and the total time the sheets are in the baths before they are ready for welding does not exceed about two minutes. Fur- .thermore, the time limits, while relatively critical for the solutions mentioned, are easy tocontrol.

The steps practiced by me for cleaning the remaining aluminum oxide in about seconds, but this time is not as critical as the time in which the sheets are subjected to the bath of the hydrofluoric acid solution.

Nothing herein shall be deemed to preclude,

the use of a suitable wetting agent, if desired, and the invention is intendedv to extend to such use.

Having described the invention, I claim:

1. The method of conditioning aluminum sheets for welding that comprises the steps of; first covering the surfaces of the sheetswith a solution of hydrofluoric acid until the oxide on the surfaces thereof is broken, then covering the surfaces of said sheets with a solution of hydrochloric acid until the solubles in said hydrofluoric acid and the remaining oxide on said surfaces are cleaned therefrom.

2. The method of conditioning aluminum sheets for welding that comprisesthe steps of first covering the surfaces of said sheets with "about a 1 /z% to 2 /2% solution of hydrofluoric acid for from about 20' to 30 seconds, then cover- .ing the surfaces of said sheets with about'a 4% v to fi'solution of hydrofluoric acid for from about 60 to seconds. J

3. The method of conditioning aluminum sheets for welding that comprises the steps of; first covering the surfaces of said sheets with substantially a 2 /2% solution of hydrofluoric acid for substantially 20 seconds, then covering the surfaces of said sheets with substantially a 6% solution of hydrochloric acid for about 90 seconds. 1 I

4. The' method of conditioning aluminum sheets for welding that comprises the steps of;

first immersing said sheets in a bath oonsistin 4 of about a 1/. to 2 /270 solution of hydrofluoric. acid for a predetermined time not exceeding substantially 30 seconds; then removing said sheets from said bath and immediately immersing them in a bath consisting of about a 4% (706% solution of hydrochloric acid for a predetermined period I of time not exceeding substantially 90 seconds.

5. The method of conditioning aluminum material for welding that comprises the stepsof; immersing such material in a bath comprising about a 1 /2 to about a 2 /2% solution of hydrofiuoric acid for a period of about to seconds; then removing said material from said bath and immediately immersing it in a second bath comprising about a 4% to about a 6% solution of a hydrochloric acid for a period of about to seconds, and thereafter removing said material from said second bath and removing therefrom such of said latter solution that may be thereon.

DON E. STEVENS. 

